Transient heat flux measurement of natural convection in an inclined enclosure with time-periodically-varying wall temperature

The transient natural convection in an inclined enclosure filled with water is studied experimentally for the time-periodically-varying wall temperature on one side wall and constant average temperature on the opposing side wall. This system has no temperature difference between the opposing two side walls in time-averaged sense. The temperatures of two opposing walls and the heat flux across the enclosure are measured by a heat flux meter. Based on the experimental results, the effects of time-periodically-varying wall temperature and inclined angles of the enclosure on heat transfer characteristics are studied. The experimental results show that, with the upper wall temperature oscillating, the heat flux across the enclosure is also periodically varied with time, and the net heat flux is from the lower wall to the upper wall. Numerical computations are also conducted and numerical results are qualitatively assured by the experimental measurements.     

Laminar natural convection in a fully partitioned enclosure containing fluid with nonlinear thermophysical properties

The effects of a heat conducting partition on the laminar natural convection heat transfer and fluid flow were obtained by comparing the numerical and experimental results for a cubic enclosure without and with a partition. The two opposite vertical walls of the enclosure were isothermal at different temperatures. The working fluid was glycerol. The complete vertical partition, made of Plexiglass, was positioned in the middle of the enclosure. The visualizations of the velocity and temperature fields were obtained by using respectively, Plexiglass and liquid crystal particles as tracers. A middle plane perpendicular to the partition was numerically modeled. The steady two-dimensional model accounted for the variable thermophysical properties of the fluid. The finite volume method based on the finite difference approach was applied. The convective terms were approximated using a deferred correction central difference scheme. The velocity and temperature fields and the distribution of the local and average Nusselt numbers were found as a function of the Rayleigh (38 000 <Ra <369 000) and Prandtl (2700 < Pr < 7000) numbers.     

Natural convection to water from arrays of short wall-attached cylinders

Experiments of natural convection from arrays of one, two, and three horizontal cylinders attached vertically one above the other to a heated, vertical flat plate in water have shown that the lowest cylinder is essentially unaffected by cylinders above it and has a heat transfer rate less than that of an infinitely long cylinder. The effect on the heat transfer from cylinders in the wake of the lowest cylinder is primarily a function of the spacing between the cylinders, with the increase being larger for greater spacing. For these wake cylinders, increases are sufficient to cause the heat transfer to equal that of an infinite cylinder.     

Unsteady Natural Convection Flow in a Square Cavity Filled with a Porous Medium Due to Impulsive Change in Wall Temperature

Unsteady natural convection flow in a two-dimensional square cavity filled with a porous material has been studied. The flow is initially steady where the left-hand vertical wall has temperature T _h and the right-hand vertical wall is maintained at temperature T _c (T _h > T _c) and the horizontal walls are insulated. At time t > 0, the left-hand vertical wall temperature is suddenly raised to which introduces unsteadiness in the flow field. The partial differential equations governing the unsteady natural convection flow have been solved numerically using a finite control volume method. The computation has been carried out until the final steady state is reached. It is found that the average Nusselt number attains a minimum during the transient period and that the time required to reach the final steady state is longer for low Rayleigh number and shorter for high Rayleigh number.     

Double diffusive natural convection in a partially heated enclosure with Soret and Dufour effects

The effect of double-diffusive natural convection of water in a partially heated enclosure with Soret and Dufour coefficients around the density maximum is studied numerically. The right vertical wall has constant temperature θ_c, while left vertical wall is partially heated θ_h, with θ_h > θ_c. The concentration in right wall is maintained higher than left wall (C_c < C_h) for case I, and concentration is lower in right wall than left wall (C_h > C_c) for case II. The remaining left vertical wall and the two horizontal walls are considered adiabatic. Water is considered as the working fluid. The governing equations are solved by control volume method using SIMPLE algorithm with QUICK scheme. The effect of the various parameters (thermal Rayleigh number, center of the heating location, density inversion parameter, Buoyancy ratio number, Schmidt number, and Soret and Dufour coefficients) on the flow pattern and heat and mass transfer has been depicted. Comprehensive Nusselt and Sherwood numbers data are presented as functions of the governing parameters mentioned above.     

Experiments in the mixed convection regime in an isothermal open cubic cavity

A detailed experimental study of the heat transfer in an open isothermal cubic cavity for mixed natural and forced convection is reported. First the experimental model with a temperature control system, the experimental procedure, and the method for determination of the convective heat losses from the experimental data are presented. After presentation of the experimental conditions for the three models of different sizes that were tested, dimensionless parameters (Gr, Re, and Nu numbers) are introduced to generalize the results. In this way, the main influence of natural convection can be separated, and, using regressional analysis, a general interdependence among Nusselt, Reynolds, and Grashof numbers can be obtained.

The local heat transfer effects are indicated by maps of local heat transfer coefficient distributions in the cavity for different orientations of the model. To reduce the effects of convective heat transfer, the influence of different types of front masks were analyzed. The usefulness of the experimental results obtained is shown by estimating the convective heat losses for the central solar receiver of the French solar power plant THEMIS.     

Natural convection flow under a magnetic field in an inclined square enclosure differentialy heated on adjacent walls

Steady, laminar, natural-convection flow in the presence of a magnetic field in an inclined square enclosure differentially heated along the bottom and left vertical walls while the other walls are kept isothermal was considered. The governing equations were solved numerically for the stream function, vorticity and temperature ratio using the differential quadrature method for various Grashof and Hartmann numbers, inclination angle of the enclosure and direction of the magnetic field. The orientation of the enclosure changes the temperature gradient inside and has a significant effect on the flow pattern. Magnetic field suppresses the convective flow and its direction also influences the flow pattern, causing the appearance of inner loops and multiple eddies. The surface heat flux along the bottom wall is slightly increased by clockwise inclination and reduced by half by the counterclockwise inclination. The surface heat flux along the upper portion of the left side wall is reversed by the rise of warmer fluids due to the convection currents for no inclination and clockwise inclination of the enclosure.     

Local mass transfer measurements in an inclined enclosure at high Rayleigh number

An electrochemical technique is used to study local mass transfer coefficients on surfaces of inclined enclosures over the range 1.1×10⁴ < Ra_H < 1.4×10¹⁰ for a nominal Schmidt number of 2280. Scaling with gcos instead of g in the Rayleigh number correlates the data well at low angles of inclination; however, as either the aspect ratio or the angle of inclination increase, the longitudinal density stratification causes the data to deviate from a power law scaling.

非定常流函数涡量方程的一种数值解法的研究

对非定常流函数涡量方程的数值求解方法进行了改进,其中流函数一阶导数即速度项采用四阶精度的Hermitian公式,对流项由一般二阶精度的中心差分提高到四阶精度离散差分,包含温度方程在内的离散方程组采用ADI迭代方法求得定常解．以无内热体及有一内热体的封闭方腔内自然对流为例,进行了不同瑞利数（Ra）条件下的数值研究．结果表明,该方法推导简单,求解精度高且计算稳定,适用于封闭腔内高瑞利数复杂混合对流的数值模拟．     

Natural-convection heat transfer in a nonuniform finite annulus with concentric heat source

In this paper, the mechanisms of natural-convection heat transfer inside a nonuniform finite annulus have been numerically investigated. The system is actually a streamlined, water-filled latex balloon with a coaxial cylindrical heating element. The balloon can be applied as a local hyperthermia treatment device for the removal of undesirable tissue if sufficiently high temperatures and preferably uniform surface heat fluxes can be maintained. A validated control-volume-based method has been employed to solve the coupled transient three-dimensional transport equations for laminar free convection. The effects of heat-source temperature distributions and device orientations on the heat transfer have been studied. Possible design improvements of this device are discussed.     

Free convection in tilted rectangular enclosures heated at the bottom wall and vented by different slots-venting arrangements

Free convection from a tilted rectangular enclosure heated at the bottom wall and vented by uniform slots opening at different walls of the enclosure was experimentally investigated. The experiments were carried out to study the effects of venting arrangement, opening ratio and enclosure's tilt angle on the passive cooling of the enclosure. The experiments were carried out at a constant heat flux of 250 W/m² and for enclosure tilt angles ranging from 0° to 180°. Three different venting arrangements of the air from the enclosure were studied: (1) top-venting arrangement, (2) side-venting arrangement, and (3) top and side-venting arrangement. Each venting arrangement was studied at different opening ratios of 1, 0.75, 0.5 and 0.25. The results showed that: (1) for top-venting arrangement, the Nusselt number decreases as the tilt angle of the enclosure increases, (2) for side-venting and side and top-venting arrangements, the Nusselt number increases as the tilt angle increases in the range [0°, 90°], then it decreases with the increase of the tilt angle, (3) for the three venting arrangements and at any tilt angle, the Nusselt number increases with the increase of the opening ratio of the slots, (4) for any tilt angle and at any opening ratio, the top and side-venting arrangement has the highest rate of cooling of the enclosure, and (5) for small tilt angles, the rate of cooling of the enclosure for top-venting arrangement was higher than that for side-venting arrangement, but with increasing tilt angle, the rate of cooling for side-venting arrangement becomes higher than that for top-venting arrangement. Correlations were developed for the three venting arrangements to predict the average Nusselt number of the enclosure in terms of the opening ratio and the enclosure tilt angle.     

用胶囊式液晶粒子同时测量流场的温度和速度

胆甾相液晶在不同温度下显示不同的颜色,这一特性已被广泛地应用于固体表面或流体的温度测量．胶囊式胆甾相液晶具有粒子形态和温度－颜色反应特性,可以用来同时测量流体的速度和温度场．作者改进了粒子测速的测试精度;还标定了色度－温度关系,获得了自然对流流场中的速度和温度场     

Effects of anisotropy on the free convection from a vertical plate embedded in a porous medium

The effects of anisotropy on the steady laminar boundary-layer free convection over a vertical impermeable surface are analysed by using the method of integral relations. If the permeability in the direction orthogonal to the plate is greater than the permeability along the plate, then there is an increase in the temperature field.     

On the reversed flow and oscillating wake in an asymmetrically heated channel

The detailed processes of flow reversal in a buoyancy-induced flow through a one-side-heated vertical channel of finite height were simulated numerically. It is of interest to note that the wake above the heated plate is oscillatory at high Rayleigh number and there exists a minimum in the transient variation of the average Nusselt number. Additionally, the predicted steady average Nusselt number and induced flow rate are correlated by empirical equations.     

Free convection from a vertical plate with nonuniform surface heat flux and embedded in a porous medium

An analysis is presented for the calculation of heat transfer due to free convective flow along a vertical plate embedded in a porous medium with an arbitrarily varying surface heat flux. By applying the appropriate coordinate transformations and the Merk series, the governing energy equation is expressed as a set of ordinary differential equations. Numerical solutions are presented for these equations which represent universal functions and several computational examples are provided.